Diffusion and consumption of methane in an unsaturated zone in north-central Illinois, U.S.A.

The distribution of CH4 in unsaturated glacial and eolian deposits adjacent to buried low-level radioactive waste was measured, and movement of the gas from the waste source was simulated using a two-dimensional finite-difference model for gas diffusion in the unsaturated zone. Mean PCH4 was greatest (1.56 Pa) in a pebbly-sand deposit 11.6 m below the land surface and 12 m from the waste, and generally decreased with increased horizontal distance from the waste. Mean PCH4 was least (0.07 Pa) at depth of 1.8 m below land surface, regardless of distance from the waste. PCH4 at the land surface averaged 0.17 Pa. Depth versus PCH4 profiles suggest consumption of both waste-produced and atmospheric CH4 in the upper unsaturated zone, presumably by methanotrophic microorganisms. Numerical simulations of methane movement support the consumption observation; inclusion of a term in the model for consumption of CH4 in the upper 2 m of the unsaturated zone resulted in simulated PCH4 within 30% of mean PCH4 at eleven of thirteen sampling locations. A similar fit of the data was obtained for only four locations when consumption was not considered. ?? 1989.